In the magnetospheric physics, the ballooning mode instability (BMI) is a disturbance of the equilibrium between outward directed forces caused by the particle pressure and the curvature drift on one hand and inward directed forces exerted by the tangential stress of the field lines on the other hand (e.g., Hameiri et al., 1991). There exists a certain analogy to the Rayleigh-Taylor instability (RTI).
A plasma and field configuration favouring the BMI is present in the near-Earth tail. This is a region in which the transition from dipole-like to tail-like field lines occur. If both the magnetic gradient as well as the pressure gradient are directed earthwards, a disturbance of the surface separating dipolar-like from tail-like field lines can develop into a wave like structure. This structure moves westwards together with the drifting ions. The azimuthal drift speed of the particles is smaller on tail-like than on dipole-like field lines. The combination of this differential particle drift and the structure leads to space charge accumulation and polarization electric field. Field aligned currents tend to remove the positive charges produced at the leading edges and the negative charges at the trailing edges of the structure. The currents are closed in the ionosphere and in the equatorial magnetosphere. Since the closure currents in the magnetosphere are opposite to the cross-tail current, they weaken or even cancel it.
In the substorm models, one of the biggest current problem is to identify the process responsible for the triggering of the expansion phase, in which part of the near-Earth cross-tail current disappears. The weakening of the cross-tail current due to BMI might explain this. The problem is that the instability criteria are still under discussion and a definite proof that they are just fulfilled at substorm onset is still lacking.
For recent references on ballooning instability, see, e.g., Liu (1997).